The present invention relates to a machine tool provided with a movable table supporting a workpiece.
A machine tool table, supporting a workpiece, must often be capable of carrying a very heavy load, and it must be easily displaced along two separate axes disposed in the plane of the table and perpendicular to each other. Sometimes the machine tool table must be displaceable along a third axis, for example when it is rotatable around an axis perpendicular to the plane of the table. At a dynamic point of view, such a table is said to be constrained to be displaced parallel to a plane. Such an arrangement of movable machine tool table is commonly in use, for example, on electrical machining apparatus, such as electrical discharge machining apparatus and electro-chemical machining apparatus. Conventionally, linear motions of the table are effected by means of an appropriate ways and co-operating slide system comprising generally ways integral with or attached to the base of the machine tool, an intermediary slide or saddle linearly displaceable along the ways and itself provided with a second arrangement of ways disposed in a direction perpendicular to the first, and a table mounted on the second ways and displaceable relative to the saddle in one direction and displaceable relative to the machine base in all directions. In such manner the tool, which in electrical machining apparatus is generally constrained to be displaced only vertically, or only vertically with lateral displacements of very small amplitude, can be disposed over any point of the plane of the movable table.
Conventional ways, although almost universally used, present several important inconveniences. First, they are costly to manufacture for many understandable reasons. Conventional ways require many machining operations and thermal treatments in order to provide accuracy and smooth functioning.
A very important inconvenience, more particularly with respect to tables which are displaced by a servo system, is the inconvenience resulting from friction.
First of all, servo motors with sufficient power are required to displace the table and, furthermore, sufficiently rigid assemblies must be used to avoid "stick-slip". "Stick-slip" is a well known phenomenon which causes a higher coefficient of friction to appear when parts capable of being slidably displaced each relative to another are at rest than when they are displaced relative to each other, such that a predetermined force is required to displace a machine tool table from its position of rest. If a ways and slide system is poorly designed, the deformation under load of the surfaces in engagement causes sudden release of the potential energy accumulated during the deformation of the surfaces which results in a poorly defined advance, for example continuous advance is transformed into an advance being effected by steps, with an accompanying lack of machining precision. Such an inconvenience has been partially eliminated in the past as a result of providing slides with linear ball bearings, but with an accompanying increase in complexity and in the accuracy with which the ways and slides must be machined, with an accompanying large increase in cost. Ball screws have also been provided, but they present the same inconvenience of complexity and high costs. There are other important inconveniences to be considered such as wear which causes an important widening of assembly clearances. Such a problem is particularly acute in some electro-erosion apparatus in which horizontal displacements of the table are continuously provided in the course of a machining operation. There are yet further problems such as, for example, providing adequate lubrication of the ways and slide assemblies, or providing accurate machining and mounting of the ways which must be perfectly parallel or perfectly perpendicular.